The present invention relates generally to a system for producing metallic material for use in a forming process. More particularly, the present invention relates to an apparatus for and method of producing a semi-solid slurry material from a molten metal under controlled cooling conditions and without stirring for application in a semi-solid forming process.
In general, the field of semi-solid processing can be divided into two categories: thixocasting and rheocasting. In the thixocasting process, also referred to as an indirect feed process, the microstructure of the solidifying alloy is modified from a dendritic form to a discrete degenerated dendritic form before the alloy is cast into a solid billet. The solid billet is then re-heated to a partially melted, semi-solid state and then cast into a mold to produce a shaped part. In the rheocasting process, also referred to as a direct feed process, a slurry is produced in a forming vessel by cooling a liquid metal to a semi-solid state while its microstructure is modified. The semi-solid slurry is then delivered as feedstock directly to a forming press to produce a shaped part.
An example of a prior art indirect feed apparatus 10 for use in a thixocasting process is illustrated in FIG. 1. Liquid molten metal alloy M is fed into a mold 12 that is surrounded by an electromagnetic stator 14. In some prior art systems, the stator 14 is replaced by a mechanical stirring device. The electromagnetic stator 14 imparts a rotating electromagnetic field to the metal alloy M as it begins to solidify within the mold 12. The electromagnetic stirring causes a type of shearing of the alloy in its semi-solid state so that the microstructure of the primary solid particles is transformed from a dendritic state into a partially dendritic state which includes globular particles suspended in a liquid eutectic phase. As the partially solidified metal alloy M exits the mold 12, it is cooled by means of a water jacket to completely solidify the alloy into a raw billet 16. The raw billet 16 may then be cut into a number of slugs 18. Before the solidified billets 16 or slugs 18 can be processed, they are transported to a processing station where they are reheated by an induction heater 20 to transform the material back into a semi-solid state. The semi-solid material is then transferred from the induction heater 20 to a die casting machine 22 where the semi-solid material is injected into a mold 24 by means of an injection mechanism 26 to form a shaped part.
The indirect feed process typically requires complex processing equipment and numerous process steps, each having a tendency to correspondingly increase equipment and operating costs. For example, the capital expenditures and maintenance costs associated with the electromagnetic stator 14 and the induction heater 20 can be substantial. Additionally, production costs can be quite high due to the numerous process steps, including the steps of stirring the alloy, handling and processing the raw billet, and the reheating the raw billets to a semi-solid state. Moreover, due to the complexity of the overall system, cycle times are quite high.
An example of a prior art direct feed apparatus 30 for use in a rheocasting process is illustrated in FIG. 2. Similar to the indirect feed process, liquid molten metal alloy M is fed into a vessel 32 which is surrounded by an electromagnetic stator 34. However, instead of forming a completely solidified billet, the direct feed process produces a partially-solidified semi-solid material that is discharged from vessel 32 into a shot sleeve 36. The semi-solid material is then injected into a mold 38 by means of an injection mechanism 40 to form a shaped part. Another example of a direct feed apparatus is disclosed in U.S. patent application Ser. No. 09/585,061, filed on Jun. 1, 2000 and entitled xe2x80x9cApparatus and Method of Producing On-Demand Semi-Solid Material For Castingsxe2x80x9d, the contents of which are incorporated herein by reference.
Although the direct feed process is somewhat less complex than the indirect feed process, the equipment and operating costs can still be substantial due to the capital expenditures and maintenance costs associated with the electromagnetic stator 34. Additionally, production costs can also be quite high due to the multiple process steps associated with producing the semi-solid material in the vessel 32, and subsequently transferring the semi-solid material into the shot sleeve 36. Moreover, cycle times associated with the direct feed process can be quite high due to the complexity of the overall system and the multiple process steps.
In prior direct and indirect feed processes, semi-solid slurry material is typically produced by stirring a molten metal while simultaneously cooling the molten metal at a relatively high rate, usually in excess of 1 degree Celsius per second. Such stirring has typically been accomplished by either mechanical stirring or electromagnetic stirring. Vigorous stirring of the molten metal causes the molten alloy to change from a dendritic microstructure to a partially dendritic, globular microstructure. The step of stirring the molten alloy during solidification was developed in response to an assumption that a fully dendritic slurry microstructure normally formed during rapid solidification is not a desirable feature and would negatively affect part quality. Instead of stirring, semi-solid slurry material has also been produced by agitating the molten metal, such as by low frequency vibration, high-frequency wave, electric shock, or electromagnetic wave. Equiaxed nucleation has also been used to produce semi-solid slurry, which typically involves rapid under-cooling and the addition of grain refiners. Additionally, Oswald ripening and coarsening has been used to produce semi-solid slurry, which involves holding the metal alloy at a steady semi-solid temperature for a long period of time.
An example of a fully solidified dendritic microstructure formed without stirring or agitation and under rapid solidification is illustrated in FIG. 3. In the early stages of semi-solid slurry formation, dendritic particles nucleate and grow as equiaxed dendrites (envision a symmetric snow flakes) within the molten metal. The dendritic particle branches grow larger and the dendrite arms coarsen so that the primary and secondary dendrite arm spacing increases. During this growth stage in the solidification process, the dendrites impinge and become tangled with the remaining liquid phase occupying the inter-dendritic volume. At this point the viscosity of the slurry increases abruptly.
In the past, it was believed that a semi-solid material formed without stirring would have a higher viscosity than a semi-solid material formed with stirring. It was also believed that higher viscosities would adversely affect die fill. It has additionally been observed that electromagnetic and/or mechanical stirring fractures the dendritic structure formed during partial solidification of the semi-solid material. Such fracturing of the dendritic structure provides a mixture of both liquid and nodular (rounded) solid particles. The mixture of particles and liquid of the stirred formation has a sufficiently low viscosity that is thought to be favorable for the semi-solid formation of shaped parts.
Although processes that utilize stirring or other forms of agitation have been found to produce adequate results, the cost and complexity of the associated equipment is relatively high, thereby having the effect of increasing capital expenditures and maintenance costs. Further, the number and complexity of the required process steps is also increased, which also has a tendency to correspondingly increase costs. Additionally, while the use of grain refiners has proven to be somewhat successful in modifying the microstructure of a metallic alloy, the costs associated with this semi-solid production method are relatively high due to the initial cost of the grain refiners and the expense associated with recycling. Furthermore, while the Oswald ripening and coarsening method has had some degree of success in the formation of semi-solid material, this method involves lengthy processing times which correspondingly increases cycle times.
Heretofore, there has been a need for an apparatus for and method of producing a semi-solid slurry material from a molten metal under controlled cooling conditions and without stirring for application in a semi-solid forming process. The present invention satisfies this need in a novel and non-obvious way.
One form of the present invention contemplates a method of producing a semi-solid material without stirring. The method comprises heating a metal alloy to form a metallic melt, transferring an amount of the metallic melt into a vessel, nucleating the metallic melt by regulating the transferring of the metallic melt into the vessel, and crystallizing the metallic melt in the vessel by cooling the metallic melt at a controlled rate to produce a semi-solid material having a microstructure comprising rounded solid particles dispersed in a liquid metal matrix.
Another form of the present invention contemplates an apparatus for producing semi-solid material without stirring. The apparatus comprises a furnace adapted to heat a metal alloy to form a metallic melt, and a temperature-controlled vessel adapted to receive and cool an amount of the metallic melt at a controlled rate to form a semi-solid material having a microstructure comprising rounded solid particles dispersed in a liquid metal matrix. The temperature-controlled vessel has a plurality of heat transfer zones, each adapted to independently control the temperature of the metallic melt disposed adjacent thereto.
Still another form of the present invention contemplates an apparatus for producing semi-solid material suitable for semi-solid forming a shaped part. The apparatus comprises a furnace adapted to heat a metal alloy to form a metallic melt, a temperature-controlled vessel having a passage adapted to receive and cool an amount of the metallic melt at a controlled rate to cause the metallic melt to crystallize and form a semi-solid material having a microstructure comprising rounded solid particles dispersed in a liquid metal matrix, and a ram displaceable along the passage to discharge the semi-solid material therefrom.
One object of the present invention is to provide an improved method of producing semi-solid slurry material for application in semi-solid forming.
Another object of the present invention is to provide an improved apparatus for producing semi-solid slurry material for application in semi-solid forming.
Further objects of the present invention will become apparent from the following description and illustrations.